PO is an important starting material in the chemical industry. It is applied in the production of polyether polyols that are used in making polyurethane plastics. Other uses of PO include the preparation of propylene glycol, propylene glycols ethers and propylene carbonate.
The traditional route for the preparation of PO proceeds via the conversion of propylene to chloropropanols (known as “chlorohydrin process”). The reaction produces a mixture of 1-chloro-2-propanol and 2-chloro-1-propanol which are then dehydrochlorinated into PO. Lime is used as chlorine absorber in this process. This method suffers from a relatively large amount of co-produced chloride salts.
In the last 25 years PO has been prepared by epoxidation with organic hydroperoxides. The hydroperoxides are produced by homogeneous oxidation of isobutane, ethyl benzene and cumene with molecular oxygen or air. Epoxidation is accomplished by either homogeneous Mo catalysts or heterogeneous Ti-based catalysts. This technology is used by Oxirane, Halcon, ARCO (isobutane and ethylbenzene), Shell (ethylbenzene), and Sumitomo (Cumene).
Although all these processes produce PO very selectively, they suffer from the drawback of producing a co-product that needs to be isolated. This co-product then needs to be sold separately (as in the case of tert-butyl alcohol or styrene) or recycled (Cumyl alcohol), to keep the process economical. Hence these processes are multistep, and require complex facilities.
In the recent past PO processes have been developed using dilute hydrogen peroxide as an alternative to organic hydroperoxides.
For instance, from WO2005000827 a process is known for the continuous epoxidation of propene with hydrogen peroxide in the presence of a titanium silicalite catalyst and a methanol solvent, wherein the catalyst is periodically regenerated by washing with a methanol solvent at a temperature of at least 100° C. and the epoxidation reaction is carried out for periods of more than 300 h between two regeneration steps. Likewise, from U.S. Pat. No. 2002004606 a process is known for the preparation of epoxides by epoxidation of olefinic compounds with hydrogen peroxide in the presence of a titanium silicalite as a catalyst. A base is introduced into the epoxidation reactor directly or as a mixture with one or more starting substances, under pH control. A pH in the range from 4 to 9.5, preferably a pH of 5 to 9.5, is established and maintained in the reaction mixture or in the starting substance containing the base. Preferably, an aqueous-organic hydrogen peroxide solution with a pH in the range from 8 to 9 is employed and the epoxidation is carried out in a fixed bed reactor. As solvent methanol is used.
A total of about 3-7 wt % of 1-methoxy-2-propanol (or propylene glycol monomethyl ether, PGME) and propylene glycol are commonly formed in the direct epoxidation of propylene in the mixed methanol and water as solvent. Moreover, the use of organic solvents such as methanol-water or acetonitrile-water systems as solvent is disadvantage because these processes need recycle of organic solvents. Such processes require operating with complex distillation setups to separate the solvents from propylene and propylene oxide.
Furthermore, processes are known that employ Mn complexes as catalysts. Mn complexes of cyclic triamines (Mn—TmTacn complexes; “TmTacn”=1,4,7-trimethyl-1,4,7, -triazacyclononane) are known as catalysts for the epoxidation of various olefins using H2O2 as oxidant.
Of particular interest is EP0618202 (corresponding to U.S. Pat. No. 5,329,024). In EP0618202 olefins such as 4-vinylbenzoic acid, styrylacetic acid, trans-3-hexenoic acid, trans-2-hexenoic acid and allyl alcohol are epoxidised by contact with a source of oxygen and a Mn complex, preferably a dinuclear manganese complex, in which the Mn is co-ordinated to a N-containing ligand such that there is a ratio of Mn to co-ordinated N atoms of 1:3. According to this reference, the epoxidation process may be conducted in an aqueous media. When the epoxidation is conducted in an aqueous media, best results are obtained on olefins with water soluble groups. According to the examples, the epoxidation may be carried out in water, using a NaHCO3 buffer with the pH adjusted to 9.0. This reference does not teach the epoxidation of propylene in water. Propylene is listed in a list separate from the olefins with water-soluble groups. Moreover, as illustrated in the attached experiments, this process utterly fails in the conversion of propylene into PO when using the recommended catalyst with the buffer in water as a solvent. A person starting from this reference, in the preparation of PO, would therefore not have considered the epoxidation of PO in water to be possible.
Another such an attempt was made in the article by Shul'pin et al. “Oxidations by the system “hydrogen peroxide—[Mn2L2O3](PF6)2 (L=1,4,7-trimethyl-1,4,7-triazacyclononane)-oxalic acid”. Part 6. Oxidation of methane and other alkanes and olefins in water” in Journal of Organometallic Chemistry 690 (2005) 4498. Epoxidation of 1-decene was tried to carry out in the presence of water using [Mn2L2O3]2+(PF6)2. However the results revealed that no epoxide was produced in the absence of acetonitrile. From the examples in the articles it is clear that only after the addition of more than 50 wt % acetonitrile the epoxidation of 1-decene was initiated.
In EP2149569, by the same applicant, a process is described for the manufacture of a 1,2-epoxide by catalytic oxidation of a terminal olefin with hydrogen peroxide wherein the catalytic oxidation is performed in a biphasic system comprising an organic phase and an aqueous reaction medium, wherein a water-soluble manganese complex is used as oxidation catalyst, wherein a terminal olefin is used with a solubility at 20 DEG C of at least 0,01 to 100 g in 1 liter water, and wherein the molar ratio of terminal olefin to hydrogen peroxide is in the range of from 1:0.1 to 1:2. The epoxidation of propylene is not specifically mentioned.
Of further interest is the article by Dirk De Vos et al, “Epoxidation of Terminal or Electron-deficient Olefins with H2O2 catalysed by Mn-trimethyltriazacyclononane Complexes in the Presence of an Oxalate Buffer”, in Tetrahedron Letters 39 (1998) 3221-3224. In this paper the authors produce a catalyst system that is soluble and active in acetonitrile. Next, it is shown that a catalytic amount of an oxalate/oxalic acid buffer strongly enhances the catalytic properties of Mn—TmTacn complexes. It is mentioned that especially terminal olefins are easily epoxidized. There is no suggestion to use this technology on propylene. In the light of the article by Shul'pin et al, and in particular with respect to the failed epoxidation of 1-decene, it would not be expected that epoxidation of propylene would be possible in the absence of acetonitrile. Moreover, when actually performing the epoxidation of propylene, a terminal olefin, using water as solvent (i.e., not diluted with acetonitrile) in the manner described in this article, the current inventors found that no soluble catalyst was prepared. In fact, the oxalate/oxalic acid buffer appeared to have adversely affected the catalyst. Thus, a very low yield of PO with respect to the used hydrogen peroxide was found.
In the prior art related to epoxidations with Mn—TMTACN complexes (Mono nuclear Mn—TMTACN complexes or dinuclear Mn complexes like [Mn2L2O3]2+(PF6)2), propylene epoxidation was not studied.
A further reference of particular interest is WO2005095370. In this reference a catalytic process for the preparation of epoxides from alkenes is described, using a combination of transition metal salt, an inorganic promoter and an organic additive in absence of solvent or in the presence of a solvent with commercially available hydrogen peroxide. Styrene, indene, cyclohexene, α-pinene, and 1,2-dihydronaphthalene, were epoxidized, typically in a mixture comprising dodecane, urea and water as the reaction medium. Moreover, epoxidation of isoprene, 1-octene, tert-4-octene and chromene was conducted in the presence of acetonitrile as organic solvent in combination with water. However, when this process was repeated for the preparation of PO, using the maximum soluble amount of urea, a low PO yield with respect to the hydrogen peroxide was found.
In this respect it should be noted that the use of acetonitrile in combination with hydrogen peroxide and/or another oxidant is not without danger. In Organic Synthesis, Coll. Vol. 7, p. 126 (1990), for instance, a clear warning against the use of organic solvents and acetonitrile in particular may be found. Thus, this article starts with cautionary note that organic-soluble peroxides may be explosive.
From the above it is clear that the industry is still looking for a commercially attractive process for the manufacture of PO, in high turnover numbers and at high selectivity, meaning free of byproducts such as diols, propylene glycol monomethyl ethers and products due to the oxidation of solvents. Moreover, this process should have a high efficiency in terms of hydrogen peroxide use. This process should also allow the use of an aqueous solvent as reaction medium (meaning water with less than 10% by volume (v %), preferably less than 5 v %, more preferably less than 1 v % of co-solvents), to avoid environmental and other problems related to acetonitrile and similar organic solvents.
The present invention overcomes these disadvantages.